Gas turbine blades operate at temperatures up to about 1500° C. They are commonly cooled by circulating air through channels in the blade. This cooling process must be efficient in order to maximize turbine efficiency by minimizing the coolant flow requirement.
Serpentine cooling circuits route cooling air in alternating directions to fully utilize its cooling capacity before it exits the blade. Such circuits have a series of channels bounded between the external airfoil walls and internal partition walls. The external walls are in direct contact with hot combustion gases, and need cooling to maintain adequate material life. The interior surfaces of the external hot walls are the primary cooling surfaces. The internal partition walls are extensions from the hot walls, and have no direct contact with the hot gas, so they are much cooler. The surfaces of the internal partition walls serve as extended secondary cooling surfaces for the external hot walls by conduction. Cooling air flows through the serpentine cooling channels and picks up heat from the walls through forced convection. The effectiveness of this heat transfer rate is inversely proportional to the thermal boundary layer thickness. Turbulators are commonly cast on the interior surfaces of the hot external walls to promote flow turbulence and reduce the thickness of the thermal boundary layer for better convective heat transfer. The high-temperature alloys used in turbine blades generally have low thermal conductivity, and therefore have low efficiency in heat transfer. To adequately cool a turbine blade, it is important to have a sufficient area of directly cooled primary surface combined with high efficiency of heat transfer.
A turbine blade airfoil has a larger thickness near the mid-chord region. In order to maintain sufficient speed of the cooling air inside cooling channels, the cooling channels near the maximum airfoil thickness become narrow. These narrow channels have small primary cooling surfaces on the hot walls, and large secondary cooling surfaces on the partition walls. The small primary cooling surfaces limit the size of the turbulators and their effectiveness. Such narrow channels do not provide efficient convective cooling.
The invention described herein increases the primary cooling surface area on the hot walls. In addition, it reduces thermal gradients between the external walls and the internal partitions, thus reducing thermal stress in the blade structure.